Support Preamble

[ysb33r]

Asciidoctor Preamble shoudl be converted as a form of frontmatter in Leanpub.

• If a preamble is found, then frontmatter,txt must be generated in a similar way as to [preface].
• Preamble will precede [dedication] section
• Standard paragraphs will be used.
• Section will be generated without a title (level-5 section)
• Preamble does not get written to Sample book.
• Premable will not appear in TOC. (unless Leanpub book is configured to show sections at all levels).